What is so great about knowing what would happen in all possible worlds?
1) If God did not have middle knowledge such as the Molinist maintains, then God, as an omniscient being, would be ignorant of an infinite number of truths, which would hardly qualify God to be omniscient. Molinism then, redounds to the glory of God over and above other views such as Open theism, Calvinism and the like.
2) WLC writes, "Once you grasp the concept of middle knowledge, I think you’ll find it astonishing in its subtlety and power. Indeed, I’d venture to say that it is one of the most fruitful theological concepts ever conceived. I’ve applied it to the issues of Christian particularism, perseverance of the saints, and biblical inspiration; Tom Flint has used it to analyze papal infallibility and Christology, and Del Ratzsch has employed it profitably in evolutionary theory."
How does God know these things?
1) It seems arbitrary for one to get worked up about how God could have middle knowledge, but to overlook how it is He can have Natural, and Free Knowledge. Moreover, as an eternal being, God never acquired any of His essential attributes like omniscience, omnipotence, omnibenevolence, and the like; so again, an arbitrary objection. With that said, what we really want is a model of how God can plausibly have knowledge at all, but especially middle knowledge. I think the trouble comes from assuming a perceptualist model of omniscience whereby God literally looks and sees (like we do), but if there isn't anything there to look at then God can't know it. This model of knowledge is terribly anthropomorphic, and luckily, there is another model of omniscience known as the conceptualist model whereby God graps all truths intuitively, and in a properly basic manner. On this model, God grasps all truths immediately without having to "look and see." There is nothing incoherent or unfamiliar about this kind of knowledge (we have it to a very small degree compared to God), and it seems that the greatest conceivable being is capable of knowing all truths in such a manner as well.
A. God's Natural Knowledge (1st logical moment) includes things that are necessarily true independent of God's willing them. For example, 2+2=4 is true independent of God's willing it to be true and so is God's knowledge of all possible worlds.
B. God's Middle Knowledge is dependent on the will of creature. Since creatures could choose differently, God's middle knowledge would be different if they were to do so. Hence, middle knowledge is not essential to God.
C. God's free knowledge includes His foreknowledge of everything that will happen. Since this thrid moment of knowledge is logically posterior to God's creative decree, what God knows wills to happen depends on the world He decrees to create. Hence, had God chosen to create a different world, then His free knowledge would have been different. Therefore, this knowledge is not essential to God either (He could have and still be God).
On Molinism, God's knowledge of what would happen in all possible worlds serves to delimit God's Natural knowledge of all that could happen. Or again, God's Natural Knowledge has as its contents all that is broadly logically possible, whereas God's middle knowledge has as its contents all that is feasible for God to actualize. Now, God's Free Knowledge, or what will happen is logically posterior to God's Middle Knowledge, and God's decree to create. Thus, God's middle knowledge grounds His Free Knowledge, but the contents of God's free knowledge also depend on what world God decrees to create. So, if God had never decreed to create, then He wouldn't have any Free Knowledge, but He would still have Natural and Middle Knowledge.